The present invention relates to a method for forming calcium carbonate, comprising a step of reacting a starting material containing calcium oxide with carbonate ions in the presence of water to produce calcium carbonate, and a step of recovering the calcium carbonate, characterized in that the formation is carried out directly from calcium oxide to calcium carbonate without intermediate stages in a three-phase precipitation where calcium oxide is hydrated to calcium hydroxide and the thus formed calcium hydroxide is carbonated to form calcium carbonate attached to a surface of calcium hydroxide particles, wherein the hydration and carbonation are carried out in immediate succession under so intensive agitation that calcium carbonate elements attached to the surface of the calcium hydroxide become detached.
According to this method, a starting material containing calcium oxide is reacted with carbonate ions in the presence of water.
The use of calcium carbonate, precipitated calcium carbonate in particular, is growing in various industries, such as the paper, plastic, and pharmaceutical industries. The intention is to form precipitated calcium carbonate (PCC) into a fine-grained, pure, and as white as possible pigment to be used for various purposes.
Numerous methods are known from before for the manufacture of PCC. Our previous patent application FI 950411 discloses that highly fine-grained PCC pigment can be produced when fine-grained lime hydrate (slaked lime) is used as starting material and crystals are allowed to grow without essentially agitating, and that the reaction can be interrupted at a certain particle size by intensive agitation.
Patent application FI 964132 further improves the said method so that, by observing the viscosity of the thus formed nucleation mass, the correct time to interrupt the growth of particles is determined.
Patent application FI 971161 again discloses how calcium hydroxide is carbonated with carbon dioxide in a mixing device having a high energy density, the energy intensity of the mixing device being higher than 1000 kW/m3 of free space of the mixing zone of the device.
Precipitation of calcium carbonate on the surface of foreign particles is described in published applications EP 0 375 683 and EP 0 604 095, among others.
The present invention is based on the idea that sequential processes, such as hydration of lime or calcium oxide and carbonation of slaked lime are carried out by using high-energy equipment where the high energy intensity inside the device replaces the mere slow diffusion process in liquid and gas. The reactions of the method are further carried out in as high a dry content as possible, whereby it is not necessary to concentrate the end product, for example, by filtering or otherwise, but the end product as such can be used, for example, as filling or coating material for paper.
According to the invention, the formation of calcium carbonate is carried out directly from calcium oxide into calcium carbonate without intermediate stages in a heterogeneous three-phase precipitation. Three-phase precipitation means here that when calcium carbonate is formed, there are present a solid phase (calcium oxide/calcium hydroxide/calcium carbonate), a liquid phase (water and, possibly, the carbonate compounds dissolved in the water), and a gas phase (carbon dioxide in carbonation). The calcium carbonate formed in the precipitation is separated to a different phase (gas phase) from where it was formed (in liquid phase; in other words, with water present). Further growth of the precipitating calcium carbonate elements is prevented because of the lack of a reactant, so that the reaction cannot continue and the growth of the particle size is stopped. In this case, the size of the formed particles can be adjusted by the feeding amount of water: the lesser the water, the smaller the particles.
The hydration of calcium oxide and the carbonation of the hydrated part are carried out immediately after one another and under so intensive agitation that the calcium carbonate elements thus created are separated from the surface of the calcium hydroxide layer. According to the invention, very small particles are instantly produced which immediately agglomerate from the initial particles of about 20 nm, forming aggregates of about 50 nm.
More specifically, the solution according to the invention is formed by the calcium oxide and water being mixed in a fluid containing a gas and a suspension of solid matter and liquid, where the amount of the gas in the fluid is at least 2 parts by volume per one part by volume of the suspension and the amount of the liquid is 1 to 20 parts per one part of solid matter.
The present invention provides considerable advantages. Consequently, the process from raw material to end product takes only a few seconds. The present invention provides a multi-purpose process which, in a new way, combines the functions of previous solutions so that the desired end product, PCC, can be manufactured in a very short residence time and a small operational content. Additionally, it was possible to eliminate intermediate storage from the process and, furthermore, it was observed that highly intensive mixing works best when the dry content of the treated material in proportion to the water is high.
In the present method, calcium carbonate is not precipitated on the surface of foreign particles, as the published applications EP 0 375 683 and EP 0 604 095 disclose. Previous publications concerning the manufacture of PCC do not show that the carbonation is started before the slaking of the lime.
The PCC particle formed as a consequence of the three-phase, heterogeneous precipitation described above is opaque (in other words, it does not give a clear direction to light) and its crystal form is vaterite. This is very well-suited for paper coating because it provides a fairly high opacity. On the other hand, homogeneous precipitation provides a crystalline product the pigment properties of which are not as good.
In the following, the invention is described in detail with reference to the appended drawings.